In the production of manganese activated zinc silicate phosphors, excess silicic acid with long blending times are used with fluxes and long firing times in order to achieve a good reaction. This phosphor requires a high manganese concentration that is necessary for high luminescence efficiency and it is difficult to achieve a good blend, especially with the zinc component. Sometimes this phosphor manufactured by these methods results in a brown or yellow discoloration and very poor fluorescent lamps. This is due possibly to minute traces of unincorporated manganese with the zinc component.
Manganese activated zinc silicate phosphors are disclosed in U.S. Pat. Nos. 2,109,984, 2,206,280, 2,210,087, 2,222,509, 2,241,030, 2,245,414, 2,247,142, 2,544,999 and 3,416,019.
Another problem with manganese activated zinc silicate phosphors is the relatively poor fluorescent lamp maintenance. That is, the light output or lumens per watt of the phosphor decreases to a greater extent during lamp life than is desirable. This invention is concerned with a manganese activated zinc silicate phosphor having improved maintenance.